1. Field of the Invention
The present invention relates to tires for truck.bus or tires for light truck. In particular, it relates to radial tires having highly improved irregular wear resistance and durability.
2. Description of the Prior Art
In general, it has been known that a tread rubber composed of styrene-butadiene copolymer rubber (SBR) is excellent in the wear resistance and irregular wear resistance under severe conditions, such as at high temperature or upon high slip, than a tread rubber composed of natural rubber. Also the former tread rubber is higher in wet skid resistance, so that this tread rubber has been broadly used for tires for passenger cars.
On the other hand, for tires for truck.bus (TB tire) or tires for light truck (LT tire), since they are thick in tread thickness, the tread rubber of natural rubber has been generally used in view of the heat generation. Recently, the radial structure having excellent moving ability and durability has been popularly used in view of safety and economy. However, the irregular wear phenomena, which has never been a problem in the tire of the conventional bias structure, have become a great problem. Particularly, when the irregular wear phenomena occurs, the wear life (primary life, wear life until the tire is reclaimed) of tires becomes very short and the vibrating ride feeling is adversely affected. Therefore, heretofore, in order to improve the irregular wear phenomena, changes in the tread pattern and in the structure for improving the uniformity of ground contact pressure at the ground contact area of the tread have been made, but the improvement has never been heretofore fully satisfied.
Accordingly, it has been considered that the tread rubber consisting mainly of SBR which has been heretofore mainly used for tires of passenger cars, is used for TB tire or LT tire, but when such tread rubber is applied to TB tire or LT tire, for example, such tread rubber is generally high in the self heat generation against the external dynamic stimulation. This is due to the fundamental property of SBR and the vulcanized rubber is liable to generate high heat, and when the tread rubber composed of SBR is used for TB tire having a large thickness of tread, a hindrance such as separation due to heat generation is caused. Furthermore, in the unvulcanized state, scorching is apt to be caused and the operability in production is very poor, so that a large amount of process oil is generally mixed in practice. However, when a tread rubber consisting mainly of SBR in which a large amount of process oil is mixed, is used, the process oil transfers into the belt portion or carcass portion of the tire during running or storage to reduce the modulus of rubber at the belt end portion of the tire or to promote the lowering of the adhesive force between the cord and the rubber. Hence, the safety and the durable life are considerably damaged, so that there are yet a large number of problems to be solved in the use of SBR for TB or LT tire treads.
The inventor has already disclosed in U.S. Patent Application Ser. No. 076,536, now U.S. Pat. No. 4,261,403 that SBR mixed with a liquid polymer having a low molecular weight, which is considered to be capable of crosslinking as a three-dimensional effective network upon vulcanizing, can be used for the tread of TB tire or LT tire. A radial tire having excellent durable life and improved irregular wear resistance can therefore be obtained. The inventor has further variously investigated in order to improve the irregular wear resistance of radial tire.
The irregular wear phenomenon of radial tire can be classified into two kinds. One is so-called shoulder stepdown wear phenomenon, wherein whole ribs in the shoulder portion of a tire or ribs in the outer side portion of the shoulder portion of a tire are worn extraordinarily faster than ribs in the center portion of the tire. The other is so-called railway wear phenomenon, wherein the edge of ribs in the center portion of a tire is worn extraordinarily fast. The inventor has found out that these irregular wear phenomena are highly influenced by the crown radius CR, which is the radius of curvature of the ground contact surface of the tread of a tire at the cross-section containing the rotation axis of the tire. That is, when CR is large, shoulder stepdown wear is small, but railway wear is large. Conversely, when CR is small, railway wear is small, but shoulder stepdown wear is very large.
Based on the above discovery, the inventor has further investigated the relationship between the shoulder stepdown wear or railway wear and the CR in a radial tire using a tread rubber consisting mainly of SBR and has found that a radial tire having a CR not less than a certain value is remarkably superior to conventional radial tires in both the shoulder stepdown wear resistance and railway wear resistance, and accomplished present invention.